Forehead sensor placement

ABSTRACT

Forehead oximetry sensor devices and methods for determining physiological parameters using forehead oximetry sensors. One method includes placing an oximetry sensor on the forehead of a patient, such that the sensor is placed on the lower forehead region, above the eyebrow with the sensor optics placed lateral of the iris and proximal the temple; and operating the pulse oximeter to obtain the physiological parameter. In one aspect, the method also includes providing and placing a headband over the oximetry sensor, or alternately, the sensor is a headband-integrated sensor. The headband has an elastic segment sized to fit around the patient&#39;s head. The headband also includes a non-elastic segment that is smaller than and attached with the elastic segment. The non-elastic segment is sized to span a portion of the elastic segment when the elastic segment is stretched. In addition, the non-elastic segment is larger than the portion of the elastic segment it spans when the elastic segment is not stretched. When the headband or the headband-integrated sensor is sufficiently tight, it delivers a pressure in the range higher than the venous pressure and lower than the capillary pressure to the forehead of the patient.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 10/678,040, filed Oct. 1,2003 now U.S. Pat No. 7,289,837.

This application claims the benefit of U.S. Provisional PatentApplication No. 60/415,468, filed Oct. 1, 2002, which application isincorporated herein by reference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates to oximetry sensors and in particular toforehead-type oximetry sensors and methods of determining physiologicalparameters using forehead oximetry sensors.

It is known that the location on a patient's body where an oximetrysensor is applied can have an effect on the estimation of aphysiological parameter that is determined using the sensor. It is alsoknown that oximetry measurements can be obtained by placing an oximetrysensor on various locations on the body of a patient, including thefingertips, the earlobe, the foot, the head and so on. In order to havea proper sensor reading, there is a need for ensuring that the sensor isapplied to an optimal location on a patient's body; a location whereoximetry signals are stable and indicative of the actual physiologicalparameter which is being monitored.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed towards forehead oximetry sensors andmethods of determining physiological parameters using forehead oximetrysensors. In one embodiment, the present invention provides a method ofdetermining a physiological parameter using a pulse oximeter. The methodincludes placing an oximetry sensor on the forehead of a patient, suchthat the sensor is placed on the lower forehead region, above theeyebrow with the sensor optics placed lateral of the iris and proximalthe temple; and operating the pulse oximeter to obtain the physiologicalparameter. In one aspect, the method also includes providing and placinga headband over the oximetry sensor. The headband has an elastic segmentsized to fit around the patient's head. The headband also includes anon-elastic segment that is smaller than and attached with the elasticsegment. The non-elastic segment is sized to span a portion of theelastic segment when the elastic segment is stretched. In addition, thenon-elastic segment is larger than the portion of the elastic segment itspans when the elastic segment is not stretched.

In another embodiment, the present invention provides a method fordetermining a location for the placement of an oximetry sensor. Themethod includes: measuring the temperature of a plurality of locationson an area of the body of a patient; dividing the temperaturemeasurements into three categories including cold, warm and hot regions,wherein hot areas correspond with areas including those over largemovable blood vessels and wherein cold areas correspond with areasincluding those susceptible to vasoconstriction; and selecting the areathat is not hot and not cold as a location for the placement of thesensor.

In another embodiment, the present invention provides a method fordetermining a location for the placement of an oximetry sensor. Themethod includes: providing a pulse oximeter having a monitor and asensor; placing the sensor on a location on the body of a patient;measuring a pulse amplitude using the sensor; comparing the pulseamplitude to a threshold; and recommending a new sensor location usingthe monitor if the pulse amplitude is lower than the threshold.

In another embodiment, the present invention provides an oximetersensor, having a substrate having a shape similar to a shape of at leasta portion of a patient's forehead and including a section adapted tosubstantially fit over a portion of a forehead of a patient. The sensorincludes an emitter disposed on the substrate at a position located onthe section and a detector disposed on the substrate at a distance fromthe emitter; and a headband for holding the substrate against thepatient's forehead, where the headband is sized to fit around thepatient's head. In one aspect, the headband includes an elastic segmentsized to fit around a patient's head; and a non-elastic segment that issmaller than and attached with the elastic segment. The non-elasticsegment is sized to span a portion of the elastic segment when theelastic segment is stretched, and the non-elastic segment is larger thanthe portion of the elastic segment it spans when the elastic segment isnot stretched. In another aspect, the headband's non-elastic segment issized to not project out from the surface of the elastic portion whenthe headband is sufficiently tight, thus indicating an adequate level oftension corresponding with delivering a pressure in the range higherthan the venous pressure and lower than the capillary pressure to theforehead of the patient.

For a further understanding of the nature and advantages of theinvention, reference should be made to the following description takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 is a diagram of a forehead oximetry sensor applied to a patient.

FIG. 2 is a diagram of a forehead oximetry sensor held to a patient'sforehead with a headband.

FIGS. 3A-C are thermal images of a person's hands and head in a warmroom and after cold room exposure for approximately 45 minutes.

FIG. 4 is a graph showing pulse amplitude signal variations due to achange in the environment's temperature obtained from various sensorslocated at various locations on a patient's body.

FIG. 5 is a diagram of the arteries in a human head.

FIG. 6 is a detailed diagram of the arteries around a human eye.

FIG. 7 is a diagram of the arteries in a human head and a preferredlocation for an oximetry sensor.

FIG. 8 is an infrared thermal image of a human head illustrating aproper sensor placement.

FIG. 9 is an assembly drawing of an embodiment of a headband-integratedsensor.

FIG. 10 is a graph of the relationship between Lag Time and PulseAmplitude.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the present invention are directed towards foreheadoximetry sensors and methods of determining physiological parametersusing forehead oximetry sensors. During oximetry, a forehead oximetrysensor 101 (e.g., such as those manufactured by Nellcor Puritan Bennett,the assignee herein), is placed on a patient's forehead, as is shown inFIG. 1. A typical pulse oximeter measures two physiological parameters,percent oxygen saturation of arterial blood hemoglobin (SpO₂ or sat) andpulse rate. Oxygen saturation can be estimated using various techniques.In one common technique, the photocurrent generated by thephoto-detector is conditioned and processed to determine the ratio ofmodulation ratios (ratio of ratios) of the red to infrared signals. Thismodulation ratio has been observed to correlate well to arterial oxygensaturation. The pulse oximeters and sensors are empirically calibratedby measuring the modulation ratio over a range of in vivo measuredarterial oxygen saturations (SaO₂) on a set of patients, healthyvolunteers, or animals. The observed correlation is used in an inversemanner to estimate blood oxygen saturation (SpO₂) based on the measuredvalue of modulation ratios of a patient. The estimation of oxygensaturation using modulation ratios is described in U.S. Pat. No.5,853,364, entitled “METHOD AND APPARATUS FOR ESTIMATING PHYSIOLOGICALPARAMETERS USING MODEL-BASED ADAPTIVE FILTERING”, issued Dec. 29, 1998,and U.S. Pat. No. 4,911,167, entitled “METHOD AND APPARATUS FORDETECTING OPTICAL PULSES”, issued Mar. 27, 1990, and the relationshipbetween oxygen saturation and modulation ratio is further described inU.S. Pat. No. 5,645,059, entitled “MEDICAL SENSOR WITH MODULATEDENCODING SCHEME,” issued Jul. 8, 1997, the disclosures of which areherein incorporated by reference in their entirety. Most pulse oximetersextract the plethysmographic signal having first determined saturationor pulse rate.

The force applied to the forehead oximetry sensor can be a factor in theproper functioning of the sensor. Generally, a headband is not requiredto be worn with a forehead oximetry sensor, when the patient's head isupright and/or well above the chest, and/or when the patient has normalvenous pressure. In certain clinical scenarios, a headband 200 isrequired to be used in conjunction with a forehead sensor 101 (e.g., anoximetry sensor), as is shown in FIG. 2. FIG. 2 shows the sensor leadsextending from the sensor (not shown) outward from beneath the headband.Such clinical scenarios include scenarios where: patient is lying downwith his/her head near or below chest level; patient is subject toelevated venous pressure; patient is diaphoretic; patient is movingexcessively, such as during exercise; as well as other scenarios wherevenous pulsations can introduce errors in oximetry calculations. Inthose scenarios, without a headband, or force on the oximetry sensor,venous pulsations could cause an incorrect interpretation of thewaveform, and therefore result in a less than accurate determination ofthe oxygen saturation and pulse rate values. The headband can be used toapply pressure to the oximetry sensor, thus reducing the effects ofvenous pulsations. When used to support an oximetry sensor, the amountof force applied by the sensor on the forehead should be greater thanthe venous pressure, but less than the arteriole pressure. Generally, agood pressure range is one where the applied pressure is higher thanvenous pressure (e.g., 3-5 mm Hg) and lower than the capillary pressure(e.g., 22 mm Hg). Preferably, this should be between 15 mm Hg and 20 mmHg in the adult patient. Exemplary headbands having a pressure ortension indicator are described in a co-pending U.S. patent applicationSer. No. 10/677,742, entitled: “Headband with Tension Indicator,” thedisclosure of which is herein incorporated by reference in its entiretyfor all purposes. As set forth in that co-pending patent application,the headband may be adjusted for use with any size wearer by using anadjustable closure mechanism, such as for example a hook and loopclosure mechanism. The user can apply a wide range of pressures to theforehead oximetry sensor depending on the amount of tension which hasbeen applied to the headband during its placement around the wearer'shead. In addition, the tension or pressure indicating headband disclosedtherein, may be used to help establish an acceptable window of pressurefor the sensor's attachment with a patient. The headband when used witha forehead oximetry sensor assists in holding the sensor in place andapplies a gentle pressure to expel any pulsating venous blood.

The inventors having conducted various physiological studies havedetermined that in addition to the possibility of needing to apply anoximetry sensor to the forehead of a patient with a certain amount ofpressure, the actual location where the forehead oximetry sensor isapplied is also a contributor to the ultimate estimation ofphysiological parameters determined using the forehead oximeter. Anexemplary forehead oximetry sensor is described in a co-pending U.S.patent application Ser. No. 10/256,245, entitled: “Stacked AdhesiveOptical Sensor,” the disclosure of which is herein incorporated byreference in its entirety for all purposes.

The physiological studies conducted by the inventors herein have notonly lead to the discovery of preferred locations for the placement of aforehead oximetry sensor, but have also discovered why the forehead andin particular the lower forehead is a preferred sensor location.

FIGS. 3A-C are thermal images of a person's hands and head in a warmroom and after cold room exposure for approximately 45 minutes. Thesethermal images show warm and cool regions of the head and hands in warmand cold room environments. A cold room environment corresponds with theenvironment of some operating rooms, whereas a warm room corresponds toother locations. FIG. 3A shows a thermal image of a person's head andhands when the person is located in a room maintained at approximately72° F. (22° C.) (warm room). As can be seen from this figure, regions302, which include the head, the fingers and the ears are warm skinregions, indicating regions where there is adequate blood perfusion andhence regions where good oximetry readings can be obtained. In contrast,FIGS. 3B-C show thermal images of the same person as in FIG. 3A, aftershe has been exposed to a cold room maintained at approximately 58° F.(14.4° C.) for approximately 45 minutes. These figures (FIGS. 3B-C) showthat after the exposure to the cold room, region 304, (the head) is theonly warm region, whereas the fingers 306, the nose 308 and the ears 310are cold, indicating regions where there is inadequate blood perfusionand hence regions where poor pulse reading are expected to occur.

FIG. 4 is a graph showing pulse amplitude signal variations due to achange in the environment's temperature obtained from various sensorslocated at various locations on a patient's body. This figure showspulse amplitudes (e.g., % IR [infrared] modulations) obtained usingfinger, ear, and forehead sensors for humans in a warm room and a coldroom. Shown in this figure are the changes in pulse amplitude caused bycold-induced vasoconstriction. As can be seen, the forehead is notsignificantly affected, while ear and fingers show a strongvasoconstrictive response, because the pulse amplitude obtained by theforehead sensor shows no significant change as the patient is moved froma warm to a cold room. The results of this graph indicate that thelower-forehead region where the forehead sensors were applied providesthe most stable pulsatile signal strength of the three sites duringvasoconstriction. Other results indicate that the head provides anearlier indication of changes in SaO₂ than other sites due to aphenomenon known as circulation delay. This phenomenon provides thathands or fingers, especially in a cold room (e.g., operating room in asurgical unit) see changes in core arterial oxygen saturation events upto a minute later than when it occurs. Clearly such delays can adverselyimpact a patient's condition.

A reason for this vasoconstrictive effect is understood by examining thearteries of the head region, as shown in FIG. 5. This figure shows thatthe external carotid artery feeds most of the head skin including theears. The lower-forehead skin is fed by the supraorbital artery, whicharises from the internal carotid artery. The external carotid arterydoes not supply the brain, and the circulation it supports shows morevasoactivity and vasoconstrictor reflexes that the circulation of thelower-forehead region. Referring to FIG. 6, it is shown that the sameinternal carotid artery source that supplies blood to the eyes and brainsupplies the skin directly above the eyebrows. The external carotidartery supplies other facial tissues. Vasoconstrictive response affectsthe internal branch of the carotid artery less than the external branchof the carotid artery. Therefore, since the lower-forehead blood flowstems from the same circulation that feeds the brain, it is lessaffected by vasoconstriction, and hence is a more stable and predictablelocation for oximetry sensor placement.

Having identified and understood why that the lower forehead region is apreferred location for placing an oximeter sensor, a preferred locationon the lower forehead is next described. A preferred sensor placementenables a sensor to optically probe arterial circulation that is fed bythe internal carotid artery. In addition, such a preferably-placedsensor probes richly perfused regions of the microvasculature, withlittle interference from larger blood vessels; and also probescardiac-induced pulsating arterial blood, with little interference fromvenous pulsations. FIG. 7 shows such a sensor location to be the lowerforehead region 702, immediately above the eyebrow 704, with both thesensor optics (i.e., emitter and detector) located lateral of the iris706 and proximal the temple. Alternately, the preferred sensor placementis one where the sensor's emitter and the detector are directly abovethe eyebrow, such that the emitter and detector are both located lateralof the supratrochlear and supraorbital arteries and medial thesuperficial temporal artery, or in other words, placing the emitter ordetector directly above the center of the eye close to the eyebrow, andthe other (detector or emitter) approximately horizontally locatedtowards the sides of the head, a few millimeters away (e.g., 2-3 mm to15 mm). Preferably, the sensor emitter or detector is placed within ±5mm of the vertical line passing through the location of the iris, morepreferably 0 mm-3 mm lateral the iris, and the other of the emitter anddetector is placed horizontally lateral this location. Preferably also,the axis connecting the sensor optics is placed within 10 mm of the topof the eyebrow, and more preferably within 5 mm. This placement site ispreferred because it experiences little vasoconstriction since thecirculation in this region is fed by the internal carotid artery. Inaddition, this region is preferred because it experiences strongpulsatile signals, with little interference from large blood vessels.

A review of FIG. 7 also shows locations that are less preferredlocations for the placement of a forehead sensor. For example, theregion of the upper or center of forehead, scalp and facial regions areless preferred region for sensor placement, because this region hassuperficial vasoactive vessels perfused with blood from external carotidcirculation. In addition, sites over large blood vessels, such as thetemporal artery are also a lesser-preferred location for sensorplacement. For sites over the large blood vessels, the SpO₂ readingsbecome unreliable when the sensor light probes large light-absorbingobjects that move or change diameter with the heartbeat, where both redand infrared light signals become similarly modulated by the highlyopaque vessels, unrelated to the oxygen saturation of arterial blood.Regions over large pulsing blood vessels, such as the temporal andproximal regions of the supraorbital arteries themselves, shouldpreferably not be used as sensor placement sites.

FIG. 8 shows an infrared thermal image of a human head illustrating aproper sensor placement. This figure shows the cooler region 802 in andaround the nose to be a less preferred location for sensor placement,because the tissue in this region has smaller pulses, and because theregion is subject to vasoconstriction. This figure also shows thatregion 804 being the warmest region is also a lesser preferred regionfor sensor placement, because the regions directly above larger vessels(hence warmer) are subject to cardio-synchronous vessel movement. Inregion 804, while the pulsatile signal strength may seem desirable, SpO₂readings could be unreliable. In contrast, region 806, which is aboveand lateral the center of either eyebrow, is a preferred sensorplacement location. As is shown in the figure, a preferred placement fora forehead sensor, such the sensor described in co-pending U.S. patentapplication Ser. No. 10/256,245, entitled: “Stacked Adhesive OpticalSensor,” is to place the emitter above and slightly lateral the iris,with the sensor cable routed back towards the ear. Accordingly, a methodfor determining a location for the placement of an oximetry sensor,includes: measuring the temperature of a plurality of locations on anarea of the body of a patient; dividing the temperature measurementsinto three categories, namely cold, warm and hot regions; rejecting thehot areas corresponding to areas over large movable blood vessels as alocation for the placement of the sensor; rejecting the cold areascorresponding to areas susceptible to vasoconstriction as a location forthe placement of the sensor; and selecting the area that is not hot andnot cold as a location for the placement of the sensor. The temperaturemeasurement apparatus can be a thermal strip that is made part of thesensor. Alternately, the temperature measurement apparatus may be a partof a sensor attachment device, such as a headband or a hat. Yetalternately, the temperature measurement apparatus may be a separatetemperature measurement device packaged with the sensor or theattachment device. Another method for identifying regions over largercardio-synchronously moving vessels is to palpate the skin; regions inwhich pulses can be felt to the touch should be avoided, while regionswith no or minimal pulsations present represent preferred locations forsensor placement.

In light of the disclosures directed to determining a proper sensorplacement location, the sensor's mechanical design itself can beconfigured for efficient locating on the forehead and above the eye.Such a design has a sensor height (or width) that is smaller than itslength. For example, a sensor with a height (or width) smaller than 5millimeters and length between 6-15 mm satisfies such a configuration.The remaining details of such a sensor are disclosed in theabove-referenced and co-pending U.S. patent application Ser. No.10/256,245, entitled: “Stacked Adhesive Optical Sensor.”

As set forth above, an attachment device is described in a co-pendingU.S. patent application Ser. No. 10/677,742, entitled: “Headband withTension Indicator.” So, in addition to above disclosures directed to theplacements of an oximetry sensor, such placements may include thepositioning of a headband device over the sensor to hold the sensor inplace on the patient's forehead and also to provide a gentle pressure tothe forehead sensor.

Alternately, the forehead sensor can be integrated with a sensorattachment device, such as a headband. FIG. 9 is an assembly drawing ofan embodiment of a headband-integrated sensor. A headband-integratedsensor provides for a more secure and stable placement of a sensor on apatient's forehead than that of two-piece device having a separatesensor and a headband, especially for patient's who move excessively,such as neonate patients. For such patients it is much easier to applyone integrated sensor, as opposed to applying a sensor and then aseparate headband over the sensor. FIG. 9 shows an oximeter sensorplaced on a substrate 902 that can be placed, adhered, or integratedinto a headband 904. In the headband-integrated embodiment, the sensoruses an emitter 906 containing two discrete wavelengths and a detector908 placed more than 2 mm away, and ideally 10 mm-15 mm from theemitter. The surface 902 can be black in order to minimize any shuntingof light between sensor and patient skin. The sensor in a headband couldbe used in conjunction with a small, portable oximeter to allow mobilityof the user during activities. Also shown in FIG. 9 is a cable 910 forproviding drive current to the LED and for providing the detector signalto the oximeter. The cable provides the electrical connection to themonitor; it also provides power for the emitter, signal carryingconductors from the detector, and shielding to protect the small signalsfrom the detector against external electrical interference.

The sensor is shown in a multi-layer structure having a face portion912. The face 912 is the surface that is placed against the patient'sskin. The face material may have an adhesive layer such as an acrylic orsynthetic rubber adhesive, or it may be without adhesive, and typicallymade from a foam PVC or foam polyurethane material. The face 912component is preferably black so as to minimize the incidence ofreflected light that does not go through the tissue. Below the facelayer 912 are two windows 914. The windows 914 are generally a clearcomponent, such as for example, a thin film or a clear molded plasticcomponent that makes contact with the skin. The thin film window may bea polyurethane or an acrylic adhesive on a polyester film. The intent ofthe window 914 is to provide an efficient optical coupling mechanismbetween the optical components (emitter and detector) and the skin.Located above the face 914, is a Faraday shield 916. The Faraday shield916 is a conductive material, for example, a copper film or copper mesh,that is electrically connected to the monitor ground to help shield thedetector from extraneous electrical interference while passing light tothe detector. Next located are the LED 906 and the detector 908. Abovethe LED and the detector is a mask layer, which may include more thanone mask layer. The mask layer 918 is generally a thin film that isintended to block light from entering the back side of the sensor, orfrom traveling directly from emitter to detector (shunt light). Thepurpose of the mask 918 is to ensure that all of the light reaching thedetector is light from the emitter that has traveled through thecapillary bed. Above the mask layer 918 is the back layer 920. The backor the top layer is the non-tissue contacting surface of the sensor.This layer may include a cosmetic finish for the sensor, which can bewhite with some printed artwork identifying the sensor. Typicalmaterials may be Velcro loop, or soft PVC foam.

FIG. 10 is a graph of the relationship between Lag Time in detecting achange in oxygenation status of a patient and Pulse Amplitude. FIG. 10shows the head to finger time delay in seconds vs. pulse amplitude in %infrared (IR) modulation for values taken in a warm room as well asthose taken in a cold room as observed in healthy volunteers. Thisfigure shows a clear clustering of the data points, where the datapoints taken in a cold room and hence indicative of vasoconstriction allshow small pulse amplitude values (e.g., less than 1.5%) and longer headto finger lag times. On the other hand, the data points corresponding tovalues not impacted by vasoconstriction (warm room data) show a smalltime lag and larger pulses. Accordingly, a method for determining alocation for the placement of an oximetry sensor includes: measuringpulse modulation value, comparing the modulation value to a threshold,and recommending a new sensor location to be chosen by the caregiver.This recommendation has particular value when the initial sensorplacement is peripherally located, such as on a finger. Therecommendation may be made by the monitor coupled with the sensor usingan algorithm being executed by the monitor and communicated to acaregiver using the monitor's audible or visual indicators.

As will be understood by those skilled in the art, the present inventionmay be embodied in other specific forms without departing from theessential characteristics thereof. These other embodiments are intendedto be included within the scope of the present invention, which is setforth in the following claims.

1. A method for determining a location for the placement of an oximetrysensor, comprising: measuring a pulse amplitude using the sensor placedon a location on a body; comparing the pulse amplitude to a threshold;and recommending a new sensor location using the monitor if the pulseamplitude is lower than the threshold, wherein the sensor is initiallyplaced on a periphery of the body and wherein the recommending comprisesrecommending a patient's head region.
 2. The method of claim 1 whereinthe recommending comprises recommending a new sensor location in theforehead region.
 3. The method of claim 2, wherein the recommendingcomprises recommending a region up to 8 mm above the eyebrow.
 4. Themethod of claim 2, wherein the recommending comprises recommending aregion within 5 mm of a line directly above the patient's iris.
 5. Themethod of claim 2, wherein the recommending comprises recommending aregion located lateral of the supratrochlear and supraorbital arteriesand medial the superficial temporal artery.
 6. The method of claim 2,wherein the recommending comprises recommending a region that is no morethan 8 mm from the eyebrow, and such that the sensor is located lateralof the supratrochlear and supraorbital arteries and medial thesuperficial temporal artery.
 7. The method of claim 2, wherein therecommending comprises recommending a region on the forehead that isabsent superficial vasoactive vessels perfused with blood from externalcarotid circulation.
 8. A system, comprising: a sensor configured to beplaced on a patient's body; and a monitor comprising a memory, whereinthe memory stores instructions executable by the monitor for evaluatinga placement of the sensor, the instructions comprising: measuring apulse amplitude with the sensor placed peripherally on the body;comparing the pulse amplitude to a threshold; and providing anindication that the sensor should be moved to a new location on thepatient's forehead if the pulse amplitude is lower than the threshold.9. The system of claim 8, wherein the new location comprises a region upto 8 mm above the eyebrow.
 10. The system of claim 8, wherein the newlocation comprises a region within 5 mm of a line directly above thepatient's iris.
 11. The system of claim 8, wherein the new locationcomprises a region located lateral of the supratrochlear andsupraorbital arteries and medial the superficial temporal artery. 12.The system of claim 8, wherein the new location comprises a region thatis no more than 8 mm from the eyebrow, and such that the sensor islocated lateral of the supratrochlear and supraorbital arteries andmedial the superficial temporal artery.
 13. The system of claim 8,wherein the new location comprises a region on the forehead that isabsent superficial vasoactive vessels perfused with blood from externalcarotid circulation.